Diagnostic-Code: smtp; 550 A URL in this email (themeforest . ne) is listed on https://spamrl.com/. Please resolve and retry
Last-Attempt-Date: Sat, 07 Apr 2018 03:10:27 -0700 (PDT)
Could you please check also with your domain&hosting provider that they are not blacklisting themeforest&Envato somehow?
Also, if you are using cPanel on your web hosting server and if you have and use Spam Assassin it is a useful idea to add there themeforest as “whitelist_from”